Along with the widespread use of a digital camera, a camera-equipped cell phone and the like, the market of the solid-state imaging apparatus has been remarkably developed. In this flow of development, the needs have changed to the development of a high resolution and high sensitivity of the solid-state imaging apparatus. In recent years, following to the development of a thin digital camera and a thin cell phone, there is an increasing need for thinning a camera part. In other words, a lens used for the camera part becomes to have a short focal length, which means that light is incident on a solid-state imaging apparatus with a wide angle (a wide angle measured from a vertical axis of an incidence plane).
In the present, in a charged-coupled device (CCD) and a metal oxide semiconductor (MOS) imaging sensor that are commonly used as solid-state imaging apparatuses, semiconductor integrated circuits having multiple light-receiving parts are arranged in a two-dimensional array, in which a light signal from an object is converted into an electric signal.
The sensitivity of the solid-state imaging apparatus is defined based on an amount of output current of a light-collecting device to the amount of incident light so that leading the incident light certainly into the light-collecting device is an important factor for the improvement of sensitivity.
FIG. 4 is a diagram showing an example of a fundamental structure of a conventional general pixel. As shown in FIG. 4, light 106 (light indicated by dashed lines) which is incident vertically into a microlens 105 is dispersed into colors using one of red (R), green (G), and blue (B) color filters 2, and then converted into an electric signal at a light-receiving device 6. Since relatively high light-collection efficiency can be obtained, the microlens 105 is used in almost all solid-state imaging apparatuses.
There is a suggestion as an example of a structure in which a lens of each pixel is asymmetrically arranged in the solid-state imaging apparatus using the micro-lenses (e.g. refer to First Patent Reference).
Further, various technologies are disclosed as a solid-state imaging apparatus using Fresnel lenses (e.g. refer to Second Patent Reference and Third Patent Reference).
In the technology disclosed in the Second Patent Reference, a lens is made up of multiple layers which have different refractive indexes in concentric circles, and the central part has the highest refractive index as the refractive index decreases towards the peripheral part of the concentric circle structure. Further, in the technology disclosed in the Third Patent Reference, a thickness distribution type lens and a distributed index type lens which has a consecutive refractive index distribution by doping are used.
Patent Reference 1: Japanese Laid-Open Patent Application No. 2001-196568
Patent Reference 2: Japanese Laid-Open Patent Application No. 2000-39503
Patent Reference 3: Japanese Laid-Open Patent Application No. 5-251673